The invention relates to coding systems for coding a wideband digital signal having first and second signal components (e.g., a wideband digital stereo signal having a left signal component and a right signal component) which each have a specific sampling frequency F.sub.s. Such coding systems comprises a subband coding unit and a quantizing unit.
The subband coding unit (a) divides the bandwidth of the wideband digital signal into M successive subbands which augment with frequency, where M is a positive integer, and (b) generates (i) M first subband signals, in response to the first signal component, and (ii) M second subband signals, in response to the second signal component, such that a first subband signal of the first subband signals and a second subband signal of the second subband signals is in each of the subbands. Each of the first and second subband signals are composed of successive signal blocks of q samples of that respective subband signal, where q is a positive integer.
The quantizing unit either (a) quantizes a first signal block of the first subband signal and a second signal block of the second subband signal in a subband SB.sub.m, where m is an integer such that 1.ltoreq.m.ltoreq.M, separately, the first and second signal blocks being time-equivalent, or (b) a composite signal block having q samples obtained by combining the first and the second signal blocks together. When the first and second signal blocks are quantized, each of the q samples in the first signal block are represented by n.sub.ml bits, and each of the q samples in the second signal block are represented by n.sub.mr bits. When the composite signal block is quantized, each of the q samples therein are represented by n.sub.mc bits.
The coding systems can also include a scale factor determining unit. The scale factor determining unit determines scale factor information relating to a scale factor SF.sub.l for the first signal block and a scale factor SF.sub.r for the second signal block.
The invention also relates to transmitters and transmitting and receiving systems which include the above-described coding system, and receivers for receiving the results of the above-described coding system. The transmitters and the transmitting and receiving systems are capable of transmitting the composite signal block and the scale factor information, and the receivers and the transmitting and receiving systems are capable of receiving the composite signal block and the scale factor information. Finally, the invention also relates to methods of performing the above-described coding.
A coding system of the type described in the opening paragraphs is known from Dutch Patent Application No. 90.00.338, to which U.S. Pat. No. 5,323,396 corresponds. The coding system described in Dutch Patent Application No. 90.00.338 enables coding of signal blocks of the first and second subband signals in some subbands in a so-called stereo intensity mode. That mode refers to the case in which a first signal block of the first subband signal and a second signal block of the second subband signal in a subband are combined to obtain a composite signal block, the first and second signal blocks which are combined being time-equivalent.
Coding in the stereo intensity mode is used when the waveform in the first and second subband signals (more particularly, the first and second signal blocks) in a subband are important, but the phase difference between those two is not important. This is the case for signals (i.e., signal blocks) in higher subbands because the human auditory system is less phase-sensitive to the frequencies in those subbands.
By implementing coding in the stereo intensity mode, a smaller information content is available for transmission (of the composite signal block representing the first and second signal blocks) than when the first and second signal blocks are coded separately, assuming the same coding accuracy. The size of the information content will be dependent on the accuracy of the coding. Better accuracy requires a large information content and vice-a-versa.
When a transmitter and/or transmitting and receiving system utilize the stereo intensity mode for coding, the composite signal block and the scale factor information relating to the scale factors for each of the first and second signal blocks (used in creating the composite signal block) are transmitted. A receiver than multiplies the samples in the composite signal block by each of those scale factors separately to obtain replicas of each of the first and second signal blocks. As a result, the respective pairs of time-equivalent samples in each of the first and second signal block replicas will differ from one another in the same manner as their scale factors differ.
The coding system described in Dutch Patent Application No. 90.00,338 is disadvantageous in that coding in the stereo intensity mode does not provide good signal transmission results. In particular, a decoded signal may have some distortion.